When the infant is suckled, afferent impulses from sensory stimulation of nerve terminals in the areolus travel to the central nervous system, where they promote the release of oxytocin from the posterior pituitary. This neuroendocrine reflex can be conditioned, and in the woman oxytocin release is often associated with such stimuli as the sight or sound, or even the thought, of the infant. The oxytocin is carried through the bloodstream to the mammary gland, where it interacts with specific receptors on myoepithelial cells, initiating their contraction and expelling milk from the alveoli into the ducts and subareolar sinuses. The passage of milk through the ducts is facilitated by longitudinally arranged myoepithelial cell processes whose contraction shortens and widens the ducts, allowing free flow of milk to the nipple. Milk is removed from the nipple not so much by suction as by the stripping motion of the tongue against the hard palate. This motion carries milk through the teat into the baby's mouth. The letdown response is decreased by psychological stress or pain, which interfere with oxytocin release. Oxytocin also appears to be involved in regulating maternal behavior in laboratory animals and may play a similar role in humans.
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